<?php
    $arr=[0,1,2,3,4,5,6,7,8,9,"a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z","A","B","C","E","F","G","H","I","J","K","L","M","N","O","P","Q","R","S","T","U","V","W","X","Y","Z"];
    $num=array_rand($arr,4);
    echo "<hr>";
    print_r($num);
    echo "<hr>";
    for($n=0;$n<count($num);$n++){
        echo $arr[$num[$n]];
    }
?>


<?php
echo "<hr>";
    for($i=0;$i<=9;$i++){
        $arr1[]=$i;
    }
    // var_dump($arr1);
    for($a="a";$a<="z";$a++){
        $arr1[]=$a;
    }
    print_r($arr1);

?>